Globular cluster Messier 69 (M69, NGC 6637), similar to its neighbor
M70, is
one of the smaller and fainter globular clusters in Messier's catalog. It can
just be seen in a dark night with a 7x50 or 10x50 pair of binoculars, if the
observing location is not too much north. However from Paris, Messier's
observing place, it is a difficult object.

M69 was discovered by Charles Messier and
added to his catalog on
August 31, 1780, the same night he found M70.
The discovery occurred when Messier was looking for a nebulous object
cataloged by
Lacaille in 1751-52 as Lac I.11;
he had already looked for that object in vain in 1764. Messier thought he had
recovered Lacaille's object and identified M69 with
Lac I.11 (NGC 6634), but this is
probably a misidentification,
as Glen Cozens of Australia has pointed out:

M69 is considerably fainter than any other Lacaille object and could
probably not be seen with Lacaille's 0.5-inch aperture telescope.

Lacaille's position deviates from M69 by an untypically large about 1.2
degrees, which cannot be easily assigned to a copy or grid error.

There is a grouping of three stars of mag 8.3, 7.8, 8.7 well at Lacaille's
position, which might have looked like a nebula in Lacaille's modest instrument.

M69 is only 7.1 arc minutes in diameter in long exposure photos,
corresponding to roughly 61 light years at its 29,700 light years distance.
Very deep photos show that it is somewhat more extended: 9.8 arc minutes, or
linearly, about 85 light years.
The visually bright compact core is less than half, only about 3'.
As M69 is quite close to the Galactic Center (only about 6,200 light-years
distant), its tidal gravitational radius is comparatively small, 8.35' or
72 light years.
Its stellar concentration is about average for a globular cluster, as it is
of concentration class V.
Its central core has a diameter of 0.68' or roughly 6 light years, while its
half-mass radius is 0.83' or about 7.2 light years.

M69's spectral type has been determined as G2 or G3, and its color index is
B-V = 1.01.
It is one of the metal-richest globulars, meaning that its stars show a
relatively high abundance of elements heavier than Helium.
Nevertheless, this value is still significantly lower than that for the
younger (Population I) stars like our Sun, indicating that even this
globular was formed at early cosmic times when the universe contained less
heavier elements, as these elements still had to be formed in the stars.

The distance of M69, about 29,700 light years, is roughly the same as that
of its apparent neighbor, M70 which is at about
29,400 light years. This indicates that these two globulars happen to be
physically neighbored; their mutual distance can be calculated to be as
small as about 1,800 light years. In contrast, the also apparently nearby
situated globular M54 is about three times as
distant.

M69 is poor in variable stars: Shapley found not a single one at all, and
the number of known variables is now still as low as 8, 2 of them being
Mira-type variable stars with periods of about 200 days.